Process for forming double-strand edged monofilament line for use in line trimmers

ABSTRACT

A process for forming flexible cutting line for use in rotary vegetation trimmers of the type having two or more monofilament lines mounted on a common spool. The line produced by the process of the present invention defines two monofilament strands joined together in a side-by-side relationship by a severable bond. Each strand defines a plurality of circumferentially spaced and outwardly projecting vegetation cutting edges thereon, two of the edges on one of the strands being in parallel contact with two of the edges on the other strand so as to define a pair of parallel readily severable welds bonding the strands together in a side-by-side disposition. The line forming process includes the steps of extruding the strands such that two of the edges on one strand are in proximate disposition and opposed parallel alignment with two of the edges on the other strand, directing the extruding strands together in edge-to-edge parallel contact along the opposed edges such that an enclosed space is defined between the strands so that the heat in the strands can create a positive pressure in the enclosed area to prevent inseparable fusion of the strands. The strands are then directed in edge-to-edge contact into a cooling quench bath, and pulled through the bath to effect crystallization and bonding together of the strands in an adjacent disposition along two readily severable parallel welds defined by the contacting edges.

BACKGROUND OF THE INVENTION

The present invention is directed to an assembly and process for formingflexible cutting line for use in rotary trimmers which is comprised oftwo monofilament strands joined by a severable bond wherein each stranddefines a plurality of raised elongated cutting edges.

Flexible line rotary trimmers are used for cutting vegetation such asgrass and weeds, particularly along walks, fences and flower beds andaround trees. These devices comprise a motor driven rotary head whichcarries one or more lengths of monofilament line mounted on a spoolwithin a housing. Extended end portions of each line project from thespool through guides in the side of the housing. As the head rotates athigh speed, the end portions of the lines are caused to projectoutwardly from the housing by the centrifugal forces acting thereon andfunction as cutting blades. The majority of trimmer heads presently inuse employ two separate monofilament nylon lines which are both mountedon a common spool and project from the spool and housing throughdiametrically opposed guides in the trimmer head housing.

The spool which carries the line is mounted within the housing such thatit rotates with the housing during use but can be selectively rotatedrelative the housing to pay out additional line when the projecting endportions of the line become worn or severed. Because these headstypically employ two separate cutting lines, and occasionally three orfour such lines, care must be taken in winding the lines about thecommon spool to prevent the lines from crossing over one another orotherwise tangling within the housing. If the lines become tangledwithin the housing, additional line cannot be payed out during use oreven pulled from the head without having to disassemble the head. Thisproblem is particularly acute in fully automatic and bump-feed headswherein even the slightest tangle can interfere with the proper indexingand paying out of the line. In addition to interfering with the properline feeding mechanisms of the flexible trimmer heads, internal tanglescan also cause balance and vibration problems which make the trimmermore difficult to use.

To solve the problem of line tangle, monofilament line has beendeveloped which comprises two strands secured together along theiradjacent lengths by a readily severable weld. The resulting doublestrand line is then simply wound about the spool and the end portionsseparated along their weld so that the end portions of the separatedstrands can be fed out through the opposed guides. Bonding the twostrands together in this manner along their entire lengths preventstangling of the strands within the housing and, if the strands areproperly joined, allows the strands to be readily separated for thefeeding of new line through the opposed guides. Such a double ordual-strand line and methods and apparatures for manufacturing such lineare disclosed in applicant's pending applications, Ser. No. 08/597,178,filed Feb. 6, 1996, and Ser. No. 08/782,333 filed Jan. 13, 1997.

In an effort to improve the cutting ability of monofilament line, theline has been extruded so as to define a plurality of raised cuttingedges extending longitudinally along the line. Different cross-sectionalconfigurations such as squares, diamonds and star shapes, have beendeveloped to provide these multiple cutting edges. In certain designs,two sets of alternating elongated edges are provided. The first setprotrudes beyond the second set and the recessed edges in the second setare disposed between the edges in the first set. Accordingly, when themore outwardly protruding edges in the first set become worn down androunded with use, the second set of sharp edges will come into contactwith the vegetation and thus effectively provide a set of fresh, sharpcutting edges with which to trim the vegetation.

Edged cutting line is becoming increasingly popular. However, such lineis subject to the entanglement problems discussed above and a method foreconomically forming a suitable dual line with edged strands has notheretofore been developed. The first dual-strand lines were formed bysecuring two strands together along their adjacent lengths by a suitableadhesive. Such line, however, was found to be excessively expensive tomanufacture and the strength of the adhesive bond between the twostrands was inconsistent and caused premature separation of the strands.As a result, such a process is equally ill-suited for use in forming adual-strand edged line.

In the forming process previously developed by applicant, which is thesubject of the above identified patent applications, the problems withadhesively formed dual-strand lines were overcome through the use of anovel extrusion process. One or more pairs of nylon strands are extrudedwith the strands in each pair being in proximate disposition. Thestrands are directed from the extrusion die into a cooling quench bathwhere the two strands in each pair are brought together on a collectingguide in a side-by-side abutting disposition to initiate the forming ofa continuous weld therebetween. The pairs of adjoining strands are thenpulled through the quench bath as the strands in each pair begin tocrystallize and bond together to form a plurality of pairs of joinedmonofilament strands. It has been found, however, that while thatprocess provides an excellent weld for monofilament strands having acircular cross-section, it is not particularly well-suited for bondingtogether pairs of nylon polymer edged strands having a plurality ofraised cutting edges extending therealong.

In the above-discussed process, the two strands are brought together ina side-by-side disposition within the cooling quench bath tosufficiently crystallize the strands prior to contact to prevent the twostrands from permanently fusing together upon contact. As the pairs ofclosely spaced molten strands extend downwardly from the extrusion dieto the collecting guide where the strands in each pair are brought intocontact below the surface of the cooling water, the individual strandstend to twist. While this is of little consequence when dealing withstrands which are circular in cross-section, if the strands defineelongated projecting ribs and one or both of the strands becomesslightly twisted, the ribs on the two strands will not align whenbrought together but cross over one another. The result will be aninconsistent weld and an unsatisfactory product. While it may bepossible to avoid such twisting with extensive care in the handling ofthe strands within the cooling bath, the care required would not becompatible with an economically viable manufacturing process andtwisting might still occur. It was discovered, however, that properalignment of the strands is not the only problem in attempting toextrude such a product.

As noted above, edged cutting line can be formed in numerouscross-sectional configurations. Depending on their shape, a pair ofedged strands could be joined together in a dual-line configurationalong a pair of abutting flat surfaces, along abutting edges or a pairof curved surfaces, or possibly along combinations of such surfaces. Ifthe strands could be joined along a rounded surface, like a conventionaldual-strand line, applicant's previously discussed extrusion processcould be employed to produce the line, provided the line twistingproblem could be solved. However, when joining the strands togetheralong a curved surface, the available remaining space for the projectingedges is quite limited and thus the number of projecting edges whichcould be formed on each strand would be very limited. In addition, it isgenerally preferred to provide a uniform distribution of the cuttingedges about the strands. As a result, the more popular edged lines aresquare in cross-section or of a diamond or of a star-like shape so as todefine multiple raised edges evenly spaced about the outer surface ofthe line. Attempting to bond a pair of edged strands together alongabutting flat surfaces, however, would result in inseparable fusion,even though the contact between the strands occurred in the quench bath,due to the large area of surface contact between the two molten strands.Attempts to bond edged strands along edge-to-edge contact using theaforesaid extrusion process would also prove unsuccessful. The contactarea between the two strands was found to be too small to provide asuitable weld unless the edges were relatively rounded. There is aperception, however, that sharp edges provide superior vegetationcutting. If the two edged strands were pressed together along theirabutting edges in an effort to strengthen the weld, there was asubstantial risk that inseparable fusion would result.

Efforts were then made using applicant's previously developed extrusionprocess to form pairs of star-type edged strands in a dual lineconfiguration along a pair of abutting edges or star points, as opposedto joining the strands in single edge-to-edge contact. The resultingconfiguration would comprise two isolated welds extending along thepoints of contact, separated by the recessed wall surfaces of thestrands. The total surface area of contact between the strands would betwice that of single edge-to-edge contact which was believed to be asufficient area of surface contact to produce a severable bond betweenthe strands of adequate strength to prevent premature separation, yetnot such a large single contact area, as in side to side contact, thatwould cause inseparable fusion. However, initial testing of the proposednew process did not produce the desired results, even when strandtwisting was carefully avoided. It was found that when the two strandsare joined along their projecting edges in the quench bath, the waterbecomes trapped between the strands in the area enclosed by the abuttingedges and the oppositely facing recessed sides of the two strands. Thetrapped water is caused to boil by the heat of the molten strands,creating an increase in pressure between the two strands which preventsthe strands from properly bonding together.

Thus, even though a very consistent, durable yet readily severable weldcan be created between two conventional nylons strands to form a highlysuccessful, tangle-resistant dual-strand monofilament cutting line, thesame result have not heretofore been obtained with strands definingmultiple raised cutting edges. The process disclosed herein achievesthat result.

SUMMARY OF THE INVENTION

Briefly, the present invention comprises an assembly and process forforming one or more lengths of double-strand monofilament edged cuttingline for use in rotary trimmers. The line is of single piececonstruction and comprises two nylon polymer strands joined together ina side-by-side disposition by a readily severable bond. Each of thejoined strands defines a plurality of outwardly projecting andlongitudinally extending edges thereon.

The process for forming this dual-strand edged line comprises theinitial step of extruding one or more pairs of molten monofilament nylonstrands wherein the strands in each pair are in proximate dispositionand each strand defines at least one recessed area disposed between apair of projecting edges. The extruded molten strands in each of theextruded pairs are directed into an abutting side-by-side dispositionsuch that the pair of the projecting edges adjacent the recessed area onone strand is aligned with and abuts the corresponding pair of edges onthe other strand. The recessed areas therebetween are in opposeddisposition, thus defining an enclosed area between the strands. Uponbeing brought into parallel edge-to-edge contact, the abutting strandsare immediately directed into and pulled along a cooling quench bath toinitiate the crystallization and bonding together of the two strandsalong the continuous welds being formed between the adjacent edges.

The pairs of joined strands are then pulled from the bath andconcurrently heated and stretched as they are pulled through a heatedoven to obtain the desired cross-sectional dimension and parallelmolecular orientation. The strands are then reheated in a relaxeddisposition as they are more slowly pulled through a second oven. Theformed double-strand edged lines are next quenched by being pulledthrough a second bath to enhance their flexibility, toughness and impactresistance and then separately spooled.

By configuring the individual extruding molten polymer strands so as todefine recessed areas between projecting edges, a wide variety ofcross-sections for edged strands can be created, including substantiallysquare and diamond shapes, in addition to a variety of star shapes. Uponbringing such strands into contact along those projecting edges, anenclosed area is provided between the strands and the two formingparallel welds. By providing an enclosed area between the strands, theheat in the molten strands will cause the air in that area to expand,generating enough outward pressure on the strands to prevent theinseparable fusion of the strands which would otherwise occur when twomolten nylon strands are brought into contact in the open air prior toany cooling and crystallization of the nylon polymer material in aquench bath. This pressure, however, is not so great as to preventformation of the desired welds along the abutting edges of the twostrands as occurred when the same configuration of edged strands werebrought together in the cooling quench bath. In addition, properalignment of the edged strands is readily achieved by bringing thestrands together above the quench bath and directly below the extrusiondie so as to prevent any twisting of the individual strands. The resultis a flexible cutting line of single piece construction comprised of twomonofilament nylon strands which define a plurality of cutting edgesthereon and are bonded together by a pair of readily severable welds.

It is the principal object of the present invention to provide a processand apparatus for economically manufacturing a flexible nylon cuttingline comprised of two edged strands joined together in a side-by-siderelationship along readily severable bond for use in flexible-linetrimmer heads of the type employing two or more cutting lines.

Other objects and advantages of the present invention will becomereadily apparent from the following detailed description taken inconjunction with the accompanying drawings.

IN THE DRAWINGS

FIG. 1 is a schematic representation of a first portion of themanufacturing process of the present invention.

FIG. 2 is a schematic representation of the remainder of themanufacturing process of the present invention.

FIGS. 3A-3D are enlarged cross-sectional views of differentconfigurations of edged strands formed in accordance with the presentinvention.

FIG. 4 is a perspective view of an end portion of a length ofdouble-strand edged line formed in accordance with the present inventionshowing the end portion thereof being separated into its componentstrands for projection through opposed eyelets in the side wall of arotary trimmer head housing.

FIGS. 5A-5D are enlarged plan views of pairs of apertures in extrusiondies employed in the present invention, illustrating the relativeorientation of the apertures and the aperture configurations producingthe strand configurations illustrated in FIGS. 3A-3D.

FIG. 6 is a further enlarged schematic representation of the initialforming and bonding together of a pair of monofilament edged strands inaccordance with the present invention.

FIGS. 7A-7D are bottom plan views of extrusion dies for use in theprocess of the present invention illustrating different hole patternsfor producing different quantities of double-strand monofilament edgedlines, FIG. 7D being enlarged slightly for clarification purposes.

FIGS. 8A-8D are enlarged cross-sectional views of pairs of bondedstrands having been extruded through the aperture pair configurationsillustrated in FIGS. 5A-5D.

FIG. 9 is an enlarged schematic view of the filter assembly, meteringpump, extrusion pot and first quench tank and illustrating the initialforming steps of double-strand edged monofilament line in accordancewith the present invention.

FIG. 10 is a perspective view of the upstream portion of the firstquench tank showing the collecting guide and basket guide mountedtherein for directing the lengths of double-strand edged line throughthe tank.

FIG. 11 is a plan view of the preferred embodiment of the collectingguide employed in the process of the present invention to produce fivepairs of joined edged strands.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings, the process of the presentinvention is schematically represented in FIGS. 1 and 2. The result ofthe process is a double or dual-strand monofilament edged line 10comprised of a pair of monofilament edged strands 12 and 14, preferablyof uniform cross-section, which are bonded together in a side-by-siderelationship along a pair of spaced-apart thin severable welds 16' and16" as seen in FIG. 4. As will be described, welds 16' and 16" areformed along pairs of abutting projecting edges 13 which extend thelength of line 10 and are on opposed sides of an enclosed area 27 formedbetween the two strands as seen in FIGS. 8A-8D. Area 27 is defined bythe opposed surfaces of the two strands which extend between welds 16'and 16" and are recessed with respect to the welds. The configuration ofthe opposed surfaces will vary depending on the cross-sectionalconfiguration of the edged strands.

While welds 16' and 16" secure strands 12 and 14 together in a paralleljuxtaposition, they are readily severed by shearing forces. Thus,pulling the extended ends of strands 12 and 14 of the line 10 will causethe strands to readily separate along the welds 16' and 16" asillustrated in FIG. 4. Accordingly, the double-strand line 10 can beeasily wound about a trimmer head spool and stored thereon without riskof strand crossover or tangling due to the continual side-by-sidesecurement of the component strands 12 and 14. Because the end portionof the line 10 can be readily separated by a shearing force into itscomponent strands along a selected length, the two strands are easilyseparated and extended from the spool in opposed directions forinsertion through opposed line guides or eyelets in the side wall of thetrimmer head housing (not shown). The readily severable bond provided bywelds 16' and 16" also allows the strands to be easily indexed throughthe opposed line guides in automatic feed and bump feed leads duringuse.

To obtain the aforesaid properties, line 10 is preferably constructed ofa nylon copolymer material such as no. 8218 manufactured by AlliedSignal, Inc. While other material compositions could be employed incarrying out the present invention, this material, which is solid phasepolymerized in a pellet configuration, has been found to produce strong,durable and impact resistant cutting strands and bonding welds havinghigh tensile and low shear strength to provide the desired featuresdiscussed above. For line 10 comprised of strands having a transversedimension (hereinafter referred to as the diameter) up to about 0.080in., a less expensive nylon homopolymer could be used such as no. 2065by Allied Signal, Inc. Acceptable results can also be obtained at alower cost in diameters over 0.080 in. by employing mixes of nyloncopolymers and homopolymers. The use of the nylon copolymer material,without the addition of any homopolymer material, however, has beenfound to provide the strongest and most durable line.

The strands 12 and 14 of line 10 can be formed in a wide variety ofedged configurations. Examples of different cross-sectionalconfigurations of edged strands which can be formed in accordance withthe present invention are illustrated in FIGS. 3A-3D. Each configurationdefines a plurality of longitudinally extending, outwardly projectingcutting edges 13 which are preferably equidistantly spaced about thecircumference of the strand. FIG. 3A illustrates a strand having asubstantially square cross-section. FIG. 3B illustrates a substantiallydiamond-shaped strand. FIG. 3C illustrates a generally cross-shapedstrand configuration marketed by Proulx Manufacturing, Inc. of RanchoCucamonga, Calif. under the trademark Crossfire. FIG. 3D illustrates agenerally star-shaped configuration of an edged strand in whichalternating edges 13A are recessed to provide a new set of sharp cuttingedges when the more outwardly projecting edges 13B become worn throughuse. These examples are not all inclusive, but merely representative ofnumerous different cross-sectional configurations of edged strands whichcould be formed in accordance with the present invention. Theconfigurations illustrated in FIGS. 3A and 3B differ from true squareand diamond-shaped configurations in that the side walls thereof areslightly concave and not flat so that the corners of the strands definethe projecting edges necessary to form line 10 as will be furtherexplained. To the naked eye, such strands appear square ordiamond-shaped.

In manufacturing the double-strand line 10 in accordance with thepresent invention, a supply of nylon copolymer material is disposed in ahopper 20 and selectively fed through an extruder 22, a screenchanger/filter assembly 24, a metering pump 26 and an extrusion die spinpack 28 disposed within pot 30. For each length of double-strand line 10to be produced, extrusion die 28 defines a pair of spaced apertures 32.The aperture configurations in extrusion die 28 which produce joinedpairs of the particular strand configurations shown in FIGS. 3A-3D areillustrated in FIGS. 5A-5D respectively. As seen therein, theconfigurations of the apertures 32 generally correspond to the finalcross-sectional shapes of the strands except for size due to the drawingdown of the extruding strands and the radial expansion of the extrudedpolymeric material due to die swell. Inwardly tapered recessed areas 31are disposed in the upper surface of die 28 about each of the apertures32 therein to enhance material flow through the die.

The phenomenon of die swell is significant in forming edged strandshaving concave surfaces and flat surfaces which are perpendicular orsubstantially perpendicular to an axis passing through the surface andthe center of the strand. To form such flat surfaces in a strandconfiguration requires a convex aperture wall in the die (or concave asviewed from the aperture) due to the swelling of the nylon material asthe material relaxes upon passing through the die apertures. Similarly,to provide a slightly concave surface in an extruded strand requires amore severe arc in the surface of the die. For example, to form theslightly concave strand walls 15, 17 and 19 in the strand configurationsillustrated in FIGS. 3A-3C requires the use of the more acutely curvedaperture sides 15', 17' and 19' illustrated in FIGS. 5A-5C respectively.As seen from comparing FIGS. 3C and 5C and 3D and 5D, the radialswelling is not significant in forming rounded or acutely angled orpointed surfaces such as edges 13 in FIGS. 3C and 13A and 13B in FIG.3D. Rounded surfaces simply become larger in diameter as a result of dieswell which can be offset, if necessary, by a proportionate reduction inaperture size or during the drawing down of the forming strands. Acutelyangled surfaces are employed to define projecting edges, as instar-shaped strands, and the radial expansion of die swell only has asignificant effect at the base of the projecting edges. The projectingedges themselves are virtually unaffected.

Examples of dies having different numbers of aperture pairs are shown inFIGS. 7A-7D. The extrusion die 28' illustrated in FIG. 7A will producetwo lengths of double-strand monofilament line 10. Die 28" illustratedin FIG. 7B will produce three separate lengths of double-strand line 10.Die 28'" illustrated in FIG. 7C will produce four separate lengths ofdual line 10, etc. It is preferable to space each pair of apertures 32substantially equidistantly apart on the die to minimize any inadvertentcontact between the pairs of strands being extruded therethrough and toavoid creating any pressure imbalances. Accordingly, the pairs ofapertures 32 are preferably aligned along a constant radius circle (aswill be explained) as opposed to being disposed in radial alignmentwherein the more outwardly disposed aperture(s) would have lessthrough-put.

The relative orientation of the apertures 32 in each pair of aperturesis very important. The apertures 32 in each pair are oriented in theextrusion die 28 such that two projecting edges 13' of one aperture arein axial alignment with two projecting edges of the other aperture andthe connecting wall surfaces 15', 17' or 19' extending between thealigned edges 13' are in opposed and recessed disposition, asillustrated in FIGS. 5A-5D. The spacing between the axially alignededges 13' of the two apertures should be between 1/16 and 3/8 of an inchapart with the preferred spacing being 1/8 inch. So aligned, the pairsof apertures are preferably positioned in the die such that they arespaced equidistantly apart and a line X (See FIG. 7D) passing throughthe centers of each pair of apertures would be tangent to a constantdiameter circle Y. Thus, in a die 28 having five pairs of apertures, apreferred embodiment (see FIG. 7D), the corresponding apertures in eachsuccessive pair would be 72° apart as measured from the center of thedie. The purpose of such orientation is to achieve edge-to-edge parallelcontact between the extruding strands along aligned pairs of projectingedges and to minimize twisting of the strands prior to crystallization,as will be explained.

As in the extrusion of single-monofilament line, the diameters of theindividual apertures 32 in die 28 should be at least fifty percentgreater than the desired final strand diameter. By way of example, inthe aperture configuration illustrated in FIG. 5C, a diameter D acrossone of the apertures of 0.265 inches is used to provide strands havingthe cross-sectional configuration illustrated in FIG. 3C andcorresponding diameters of both 0.095 and 0.105 inches. Variations instrand size can be obtained with a given aperture size by regulating themetering pump 26 and the line speed as in the manufacture ofconventional single-strand monofilament line.

In forming line 10, the nylon polymeric material is first extrudedthrough the pairs of apertures 32 in extrusion die 28, forming acorresponding number of pairs of molten monofilament strands 12 and 14.Each pair of molten strands is directed downwardly from die 28 to acollecting guide 29 which is disposed slightly above the surface of thewater within a quench tank 34. At the collecting guide 29, the moltenstrands are aligned and pressed gently together along their opposedprojecting edges 13 to initiate the bonding together of the two strands12 and 14 along those edges. Each pair of abutting strands is thendirected in a side-by-side disposition and edge-to-edge contactdownwardly into the quench tank 34.

Quench tank 34 is filled with water which is maintained within the rangeof about 40° to 100° F., depending on the material being used, to effectcrystallization of the nylon strands as they pass through the coolerwater. If the line 10 is being constructed of the preferred nyloncopolymer no. 8218 identified above, the water in tank 34 should bemaintained within the range of 40° to 80° F. For smaller diameter lineconstructed of this material, such as 0.065-0.080 in., the watertemperature should preferably be about 60° to 70° F. and most preferablyat about 60° F. For larger diameter line such as 0.095-0.105 in. thewater temperature should preferably be at about 70° to 80° F. and mostpreferably about 70° F. If the line were being formed of a nylonhomopolymer, the water temperature need not be quite as cool as nylonhomopolymers crystallize more quickly. For example, if the aforesaidnylon homopolymer No. 2065 were being used, the water should bemaintained from about 70° to 100° F. Because this material crystallizesquickly, a preferred water temperature of 70° to 80° F. is employed withsmall diameter line such as 0.065-0.080 in. and most preferably thetemperature should be about 70° F. With larger diameter line such as0.095-0.105 in., the water temperature should be about 80° to 100° F.and most preferably about 80° F. A water cooling apparatus (not shown)is employed in tank 34 to maintain the water at the desired quenchingtemperature.

The collecting guide 29 (see FIGS. 11 and 12) is mounted on the sides ofquench tank 34 so as to be positioned directly below die 28 and isspaced about 8 to 9 inches below the die. In the configuration shown inthe drawings, collecting guide 29 comprises a pair of roller bars 29Aand 29B which are in horizontal alignment and spaced slightly apart. Thebars are carried by lateral mounting brackets 47 and a plurality ofraised ridges 31 are provided on each of the bars 29A and 29B so as todefine a series of finger guides separating the individual curvilinearguide surfaces 29', 29", 29'" etc., on bars 29A and 29B and joiningtogether and aligning the pairs of strands about one of the guidesurfaces. As each pair of strands is directed from die 28 about one ofthe guide surfaces, e.g., 29' and between a pair of the spaced ridges 31separating the individual guide surfaces, the ridges cause the strandsin each pair thereof to be pressed gently together at 33 to initiate theformation of the welds 16' and 16" therebetween. Because the apertures32 are configured and aligned as above-described, the two strands 12 and14 in each pair will abut one another along their projecting edges 13,forming welds 16' and 16" which collectively bond together the abuttingedges of the two strands. The collecting guide 29 is adjustably mountedso as to bring the strands together between approximately 1.0 to 0.25inches above the upper surface of the cooing bath in tank 34.

As the molten strands 12 and 14 are brought together in edge-to-edgecontact at 33 adjacent collecting guide 29, each pair of strands definesan enclosed area 27 between the two strands. The areas is bordered bythe abutting edges 13 and the recessed, oppositely facing side walls 15,17 or 19 of the two strands, as seen in FIGS. 8A-8D. The air entrappedin the enclosed area 27 between the two strands during the joiningtogether of the strands is rapidly heated by the molten material,creating an increase in pressure between the abutting strands. Thispressure build up urges the strands apart and prevents the strands frompermanently fusing together.

The extruded pairs of joined strands are then directed in side-by-sidedisposition from collecting guide 29 directly into the cooling quenchbath. The strands are pulled down to and about a guide 35 and a seriesof rollers 37 which are mounted in a spaced curvilinear disposition in abasket or carriage 38 adjustably mounted in the lower upstream end ofthe quench tank 34. Guide 35 is of the same configuration as guide bars29A and 29B and defines a plurality of raised annular ridges 36 thereonwhich are spaced apart and define a second series of finger guides formaintaining the alignment of the pairs of joined strands in a spacedparallel array. The finger guides defined by both the first and secondroller guides 29 and 35 could, of course, be formed by other means suchas a plurality of annular or arcuate channels formed in a solid barextending across carriage 38. The pairs of joined strands extend fromcarriage 38 in parallel alignment proximate the bottom of tank 34 andabout a second plurality of rollers 39 mounted on a second carriage 42.The last roller 39' on carriage 42 preferably defines a guide and is ofthe same configuration as guides 29 and 35. From carriage 42, the pairsof strands are directed upwardly out of the quench tank 34, and througha sponge assembly 41 which strips excess water from the strands and isprovided with a comb guide to maintain the alignment of the joinedstrands to a first roll stand 44. A roll stand 44 pulls the parallelarray of the forming lengths of double-strand line 10 from the extrusiondie 28 through the quench bath 34 and cooperates with a second rollstand 46 and a third roll stand 48 to move the lengths of line 10through the forming process as will be explained.

An adjustable mounting of collecting guide 29 on tank 34 is provided toenable one to vary slightly the vertical elevation of the guide abovethe surface of the quench water to adjust the strength of the welds 16'and 16" for differently configured and sized strands. For strands havingthe configuration illustrated in FIGS. 3C and 8C, a diameter of 0.095inches and being formed with the nylon copolymer No. 8218 identifiedearlier herein, positioning guide 29 such that the strands are broughtinto contact at about 0.5 inches above the surface cooling bath providesideal welds. For smaller sized strands this spacing could be increasedslightly and for larger strands the guide 29 could be adjusted to movethe contact points 33 slightly closer to the water. This adjustabilitycan be provided by a pair of flat bars 47 vertically mounted within tank34 adjacent the opposite side walls thereof. Bars 47 are provided with apair of closely spaced apertures 47' or aligned slots which are adaptedto receive threaded fastening members whereby the guide 29 can besecured in place at the desired elevation on the tank. Other adjustablemounting means could, of course, also be employed.

The carriages 38 and 42 are similarly adjustably mounted in tank 34 toaccommodate changes in the temperature of the cooling bath and differentline sizes. Smaller line will quench more quickly to obtain the desiredresults. Raising the carriages within the tank will shorten the quenchtime. The water within the tank is also heated by the semi-moltenstrands moving therethrough. As the temperature of the water increases,the carriages 38 and 42 can be lowered slightly, if necessary, toprovide comparable cooling of the strands.

It should also be noted that different configurations of collectingguide 29 could also be employed. The collecting guide 29 illustrated inFIGS. 10 and 11 is particularly configured so as to cooperate with a dieconfiguration having five pairs of apertures 32 to minimize any twistingof the freshly extruded molten strands as the strands pass from theextrusion die in closely spaced parallel disposition to an abuttingdisposition adjacent one of the guide surfaces 29' on the collectingguide 29. As seen in FIG. 11, five pairs of strands 12 an 14 are shownas being collected on guide 29. As can also be seen from the fiveaperture pair configuration in die 28 shown in FIG. 7D, a minimalreorientation of the strands is needed to direct the strands from theaperture alignment in die 28 to the respective guide surfaces oncollecting guide 29 shown in FIG. 12. The two parallel rollerconfiguration of collecting die 28 could also be employed in extrudingtwo and three pairs of strands. In a die having four pairs of apertures(see FIG. 7C), a differently configured collecting guide might bepreferable to the configuration shown in the drawings to minimize strandtwisting. Such a guide configuration could comprise, for example, fourangularly spaced collecting guide rollers.

As the bonding pairs of strands extend from the collecting guide 29 intothe quench bath and about guide 35 and rollers 37, the ridges 36 on theguide 35 maintain each pair of aligned and joined crystallizing strandstogether in edge-to-edge contact as they pass thereover. Thus, as rollstand 44 continually pulls the pairs of adjacent strands through thequench bath, the strands will continue to crystallize and the welds 16'and 16" will continue to form at points 33 and thus extend along andcontinuously bond together the pairs of strands 12 and 14 to form thelengths of double-strand line 10. Cross-sectional views of these bondedstrands 12 and 14 (having configurations corresponding to the dieapertures illustrated in FIGS. 5A-5D) are shown in FIGS. 8A-8D. Becausethe pairs of strands will continue to come together at points 33 andform continuous welds 16' and 16", it is not necessary to press the twostrands together down stream of collecting guide 29. Guides 35 and 39are employed to maintain the parallel alignment of the formed dual linesas they pass through the quench tank 34.

Roll stand 44, which pulls the joined pairs of strands through thequench bath, comprises an elevated drive roller 50 and a pinch roller 52for pulling the joined strands upwardly from tank 34, and two rows ofvertically and laterally spaced additional drive rollers 54 whichcooperate with rollers 50 and 52. The drive rollers in each of the threeroll strands are preferably constructed with a stainless steel outersurfaces, while the pinch rollers preferably have a hard rubber surfaceto provide the desired gripping and durability characteristics. As seenin FIGS. 1 and 9, the parallel array of spaced lines 10 extend from tank34 between drive roller 50 and pinch roller 52, downwardly therefrom andabout the two rows of drive rollers 54 and laterally therefrom into afirst oven 56. The second roll stand 46 is disposed downstream of oven56 and is comprised of two rows of vertically and laterally spaced driverollers 58 and a pinch roller 59. Roll stand 46 pulls the parallel arrayof lines 10 from the first roll stand 44 and through oven 56.

To obtain the desired physical properties in line 10, it is importantboth to stretch the line while it is being heated in oven 56 and toobtain the desired degree of crystallization of the nylon polymermaterial prior to heating and stretching. Stretching the line during theheating step provides parallel orientation of the molecular structurewithin the two strands 12 and 14 of line 10 and is achieved by providinga differential between the rotational velocities of the drive rollers 50and 54 in the first roll stand 44 and the drive rollers 58 in the secondroll stand 46. All of the drive rollers in the three roll stands arepreferably of the same size. Accordingly, by rotating the drive rollers58 in the second roll stand 46 more rapidly than the drive rollers 50and 54 in the first roll stand 44, the lines 10 are stretched as theyare pulled through oven 56.

The amount of crystallization which occurs in the molten strands priorto heating and stretching is a function of the particular material beingused, the temperature of the quench water and the quench time (timeduring which the line is submerged in the quench tank). The quench timedepends on the velocity at which the lines are pulled through the tankand the length of underwater travel. From a commercial standpoint, it isdesirable to maximize line output per unit time. This is preferablyachieved in the present invention by extending the length of the quenchtank 34 which allows the roll stands to operate at higher rotationalvelocities without decreasing quench time. It has also been found to bedesirable to operate the roll stands at constant velocities and thusvariations in the line material can be most easily accommodated byvariations in the temperature of the quench water.

By way of example, when using a ten hole die to extrude concurrentlyfive pair of double-strand lines 10 using the aforesaid preferred nyloncopolymer No. 8218, wherein the strands 12 and 14 are 0.080 inch indiameter, the diameters of die holes 32 are each 0.176 inch. The watertemperature in tank 34 is preferably 60° F. The vertical spacing betweenthe lower face of the extrusion die 28 in pot 30 and the surfaces of thewater in tank 34, which is referred to as the air gap, is about 4 to 9inches depending on the viscosity of the material used, the diameter ofthe strands being extruded and the draw-down ratios employed. The largerthe diameter of the strands being formed and the less the draw-downratio, the shorter the air gap. More viscous melts will require largerair gaps. To form welds of desired strength between the strands, thestrand contact points 33 are each about 0.5 inches above the surface ofthe water as noted earlier herein. For very light welds between thestrands, this depth could be decreased to about 0.25 inches. The melttemperature is about 420° to 480° F. and the quench tank 34 is about sixfeet in length by three feet in depth. The rotational velocity of thedrive rollers in the first roll stand 44 is 44.1 feet per minute. Toprovide the proper orientation of the molecular structure of the twostands in each length of line 10 so as to achieve the desired linestrength and durability characteristics, oven 56 is maintained at about580° F. and the ratio of the relative rotational speeds of the driverollers 58 in the second roll stand 46 to the speed of the drive rollers50 and 54 in the first roll stand 44 is about 3.1 to 1. Accordingly, therotational speed of the drive rollers 50 and 54 in the second roll stand46 in the present example is 136.71 feet per minute.

Orienting the strands by aforesaid stretching and heating placesconsiderable stress on the strands. To provide the desired strength anddurability in the final product, it is desirable to relieve this stress.This is accomplished in the present invention by subjecting the pairs ofjoined strands 12 and 14 to a second heating step. In the second heatingstep, however, the joined strands are in a relaxed state as opposed tobeing stretched during the first heating step. To provide the secondheating step, a second oven 60 is disposed downstream of the second rollstand 46. The third roll stand 48 is positioned downstream of the secondoven 60 to pull the lengths of line 10 through oven 60. Roll stand 48preferably comprises a pinch roller 62 and three drive rollers 64vertically and horizontally spaced apart as shown in FIG. 2. To pull thelines 10 through oven 62 in a relaxed state, the drive rollers of rollstand 48 are rotated at a rate about two to ten percent slower than thedrive rollers 58 of the second roll stand 46. Thus, in the aboveexample, rollers 58 would be driven at 133.97 feet per minutes. Thesecond oven 60 is maintained at a slightly lower temperature than oven56, preferably about 540° F.

A second quench tank 66 is disposed downstream of the third roll stand48 to moisten the monofilament line prior to spooling as spooled line isinhibited from absorbing from moisture in the air which is desirable infreshly extruded nylon line from a strength standpoint. A suitable lineguide 68 is provided in the lower portion of quench tank 66 to define anunderwater path for the line through tank 66, the water in tank 66 ismaintained at about the same temperature as the water in tank 34 to coolthe formed line prior to spooling. Finally, a conventional spoolingassembly 70 is deployed in the assembly line downstream of quench tank66 wherein each of the individual double-strand lines 10 formed by theaforesaid process are individually wrapped about separate spools 72 forstorage and shipment. In the example set forth above, five separatespools would be wound with line 10 by assembly 70 and a total of 199.5pounds of line 10 would be produced per hour, based on 402 feet of lineper pound.

Various changes and modifications may be made in carrying out thepresent invention without departing from the spirit and scope thereof.Insofar as these changes and modifications are within the purview of theappended claims, they are to be considered as part of the presentinvention.

I claim:
 1. A process for forming a flexible cutting line comprised oftwo edged monofilament strands joined together in a side-by-sidedisposition by a pair of spaced-apart readily severable welds for use inrotary vegetation trimmers, said process comprising the followingsteps:extruding a pair of molten monofilament strands such that eachstrand defines a plurality of circumferentially spaced and outwardlyprojecting edges thereon extending longitudinally along the strand, twoof said edges on one strand being in proximate disposition and opposedparallel alignment with two of said edges on the other strand; directingsaid strands together into edge-to-edge parallel contact along saidopposed edges such that a space is defined between said strands and saidopposed edges thereof; directing said strands in edge-to-edge contactinto a cooling quench bath; and pulling said strands in edge-to-edgeparallel contact through the bath to effect crystallization of the twostrands and the bonding together thereof along two readily severableparallel welds.
 2. The process of claim 1 including the additional stepsof concurrently stretching and heating the bonded strands and thenheating the bonded strands in a relaxed disposition.
 3. The process ofclaim 1 including the additional steps of pulling said pair of bondedstrands from the quench bath, directing said pair of bonded strands fromsaid quench bath to a heated oven at a first velocity; and pulling saidstrands through said oven at a second velocity, said second velocitybeing greater than said first velocity and then heating the bondedstrands in a relaxed disposition.
 4. A process for forming a flexiblecutting line comprised of two monofilament strands having substantiallysquare or diamond-shaped cross-sections and being joined together in aside-by-side disposition by a pair of spaced-apart readily severablewelds for use in rotary vegetation trimmers, said process comprising thefollowing steps:extruding a pair of molten monofilament strands suchthat each strand defines four outwardly projecting elongated edges andfour concave wall surfaces extending between said edges and along saidstrand, two of said edges on one strand being in proximate dispositionand opposed parallel alignment with two of said edges on the otherstrand; directing said strands together in edge-to-edge parallel contactalong said opposed edges such that a space is defined between saidstrands by said contacting edges and the concave wall surfaces extendingtherebetween; directing said strands in edge-to-edge contact into acooling quench bath; and pulling said strands in edge-to-edge parallelcontact through the bath to effect crystallization of the two strandsand the bonding together thereof in an adjacent disposition along tworeadily severable parallel welds.
 5. The process of claim 4 includingthe additional steps of concurrently stretching and heating the bondedstrands and then heating the bonded strands in a relaxed disposition. 6.The process of claim 4 including the additional steps of pulling saidpair of bonded strands from the quench bath, directing said pair ofbonded strands from said quench bath to a heated oven at a firstvelocity; and pulling said strands through said oven at a secondvelocity, said second velocity being greater than said first velocityand then heating the bonded strands in a relaxed disposition.
 7. Aprocess for forming a flexible cutting line comprised of twomonofilament strands having generally cross-shaped cross sections andbeing joined together in a side-by-side disposition by means ofspaced-apart readily severable welds for use in rotary vegetationtrimmers, said process comprising the following steps:extruding a pairof molten monofilament strands in proximate disposition such that eachstrand defines four pairs of outwardly projecting edges, four concavewall surfaces, one of said concave wall surfaces extending between thetwo edges in each of said pairs and a plurality of recessed wallsurfaces, said recessed wall surfaces extending between edges indifferent pairs, one of said pairs of edges on one strand being inproximate disposition and opposed parallel alignment with one of saidpairs of edges on the other strand; directing said strands together inedge-to-edge parallel contact along said opposed edges such that a spaceis defined between said strands by said contacting pairs of edges andthe concave wall surfaces extending therebetween; directing said strandsin edge-to-edge contact into a cooling quench bath; and pulling saidstrands in edge-to-edge parallel contact through the bath to effectcrystallization of the two strands and the bonding together thereofalong two readily severable parallel welds.
 8. The process of claim 7including the additional steps of concurrently stretching and heatingthe bonded strands and then heating the bonded strands in a relaxeddisposition.
 9. The process of claim 7 including the additional steps ofpulling said pair of bonded strands from the quench bath, directing saidpair of bonded strands from said quench bath to a heated oven at a firstvelocity; and pulling said strands through said oven at a secondvelocity, said second velocity being greater than said first velocityand then heating the bonded strands in a relaxed disposition.
 10. Aprocess for forming a flexible cutting line comprised of twomonofilament strands having generally cylindrical body portions and aplurality of circumferentially spaced and outwardly projecting edgesextending longitudinally along the strands and wherein said strands arejoined together in a side-by-side disposition by a readily severablebond for use in rotary vegetation trimmers, said process comprising thefollowing steps:extruding a pair of monofilament strands such that eachstrand defines a plurality of acutely angled and outwardly projectingedges thereon, two of said edges on one of said strand being inproximate disposition and opposed parallel alignment with two of saidedges on the other strand; directing said strands together inedge-to-edge parallel contact along said opposed edges such that anenclosed space is defined between said strands; directing said strandsin edge-to-edge contact into a cooling quench bath; and pulling saidstrands in edge-to-edge parallel contact through the bath to effectcrystallization of the two strands and the bonding together thereof inan adjacent disposition along two readily severable parallel weldsdefined by said contacting edges.
 11. The process of claim 10 includingthe additional steps of concurrently stretching and heating the bondedstrands and then heating the bonded strands in a relaxed disposition.12. The process of claim 10 including the additional steps of pullingsaid pair of bonded strands from the quench bath, directing said pair ofbonded strands from said quench bath to a heated oven at a firstvelocity; and pulling said strands through said oven at a secondvelocity, said second velocity being greater than said first velocityand then heating the bonded strands in a relaxed disposition.
 13. Aprocess for forming a flexible cutting line comprised of two edgedmonofilament strands joined together in a side-by-side disposition by apair of spaced-apart readily severable welds for use in rotaryvegetation trimmers, said process comprising the followingsteps:extruding a pair of monofilament strands in proximate dispositionsuch that each strand defines a plurality of circumferentially spacedand outwardly projecting edges thereon extending longitudinally alongthe strand, two of said edges on one strand being in opposed parallelalignment with two of said edges on the other strand and spaced fromsaid other edges a distance of about 1/8 to 3/16 of an inch; directingsaid strands together into edge-to-edge parallel contact along saidopposed edges at a predetermined location such that a space is definedbetween said strands and said opposed edges thereof, whereupon the heatin said strands creates a positive pressure in said space preventinginseparable fusion of said strands; directing said strands inedge-to-edge parallel contact into a cooling quench bath disposed adistance of between about 0.250 to 1.0 inches below said predeterminedlocation; and pulling said strands in edge-to-edge parallel contactthrough said bath to effect crystallization of the two strands and thebonding together thereof along two spaced-apart and readily severableparallel welds.
 14. A process for forming a flexible cutting linecomprised of two edged monofilament strands joined together in aside-by-side disposition by a pair of spaced-apart readily severablewelds for use in rotary vegetation trimmers, said process comprising thefollowing steps:extruding a pair of monofilament strands such that eachstrand defines a plurality of circumferentially spaced and outwardlyprojecting edges thereon extending longitudinally along the strand, twoof said edges on one strand being in proximate disposition and opposedparallel alignment with two of said edges on the other strand; directingsaid strands together into edge-to-edge parallel contact along saidopposed edges at a given location such that a space is defined betweensaid strands and said opposed edges thereof, whereupon the heat in saidstrands creates a positive pressure in said space preventing inseparablefusion of said strands; directing said strands in edge-to-edge parallelcontact into a cooling quench bath disposed a distance of between about0.250 to 1.0 inches below said predetermined location; pulling saidstrands in edge-to-edge parallel contact through said bath to effectcrystallization of the two strands and the bonding together thereofalong two spaced-apart and readily severable parallel welds; thenconcurrently stretching and heating the pair of bonded strands; and thenheating the pair of bonded strands in a relaxed disposition.
 15. Theprocess of claim 14 wherein said stretching and heating step comprisespulling said pair of bonded strands from the quench bath, directing saidpair of bonded strands from said quench bath to a heated oven at a firstvelocity; and pulling said strands through said oven at a secondvelocity, said second velocity being greater than said first velocity.16. The process of claim 15 wherein said second velocity is about threetimes greater than said first velocity.